Studies of the interaction of tritiated parathyroid hormone with the kidney are being conducted. These experiments range from injection of the radioactive hormone into the renal artery of living animals to studies of solubilized membrane receptors from this tissue. The work is designed to ask the following questions: (a) Can uptake of hormone be demonstrated in the intact kidney and what are the features of such an uptake? Is the hormone metabolized by this tissue and if so, what is the chemical and biological nature of the metabolites? (b) What is the biochemical nature of the receptor binding of this hormone to isolated plasma membranes and how is it related to activation of adenylate cyclates? Do isolated membranes metabolize PTH and if so, what is the biochemical nature of such metabolites? (c) Do solubilized receptors from kidney plasma membranes retain the features of the membrane receptors and can such receptors be purified? How are the phenomena observed in membranes (above) related to the soluble proteins obtained by detergent extraction of such membranes. Can specific functions be identified with specific proteins? The total program thus seeks to delineate the biochemical details of interaction of parathyroid with one of its target tissues from the point of entry of the hormone into the tissue through initiation of its biological function to its ultimate fate in this tissue.